Renal · Year 2 · from Renal

Case 2: Uric Acid Nephrolithiasis

Patient Presentation

Demographics: 54-year-old male

Chief Complaint: Recurrent kidney stones

History of Present Illness: The patient has had 4 kidney stones in the past 3 years. His stones have been described as "hard to see on X-ray" and are usually treated conservatively. He has metabolic syndrome with obesity, type 2 diabetes, and hypertension. He reports low urine output and dark urine.

Past Medical History:

  • Type 2 diabetes mellitus (HbA1c 8.2%)
  • Obesity (BMI 34)
  • Hypertension
  • Gout (1 episode)

Physical Examination:

  • Blood pressure: 142/88 mmHg
  • BMI: 34
  • General: Obese male
  • Abdomen: Soft, non-tender

Workup and Results

Laboratory Studies:

  • Creatinine: 1.2 mg/dL
  • Uric acid: 8.8 mg/dL (elevated)
  • Fasting glucose: 156 mg/dL

Urinalysis:

  • pH: 5.0 (acidic)
  • Specific gravity: 1.028 (concentrated)

24-hour Urine:

  • Volume: 1.2 L (low)
  • pH: 5.2 (consistently acidic)
  • Uric acid: 950 mg/day (elevated)
  • Citrate: 220 mg/day (low)

CT Scan:

  • 8 mm left renal pelvis stone
  • Hounsfield units: 280 (radiolucent - uric acid)

Clinical Image

CT scan showing a uric acid stone in the renal pelvis. Uric acid stones are radiolucent on plain X-ray but visible on CT with lower Hounsfield units (typically <500 HU) compared to calcium stones.

Diagnosis

Recurrent Uric Acid Nephrolithiasis with Metabolic Syndrome

Key features:

  • Radiolucent stones on plain X-ray (low HU on CT)
  • Persistently acidic urine (pH <5.5)
  • Metabolic syndrome with insulin resistance
  • Hyperuricosuria

Discussion

This case demonstrates uric acid stone pathophysiology:

  • Primary Risk Factor: The lecture emphasizes that acidic urine (pH <5.5) is the primary risk factor for uric acid stone formation, even more important than hyperuricemia. Uric acid has a pKa of 5.5, meaning it is protonated and insoluble below this pH.
  • Metabolic Syndrome Connection: The lecture describes how metabolic syndrome and insulin resistance impair renal ammoniagenesis, resulting in persistently acidic urine. This patient exemplifies this association.
  • Radiolucent Stones: Uric acid stones are radiolucent on plain X-ray but visible on CT with lower Hounsfield units (<500 HU) compared to calcium stones.
  • Medical Dissolution: Unlike calcium stones, uric acid stones can be dissolved by urinary alkalinization to pH 6.0-6.5.

Treatment Plan

  1. Urinary Alkalinization (Primary Therapy):
  • Potassium citrate 20 mEq three times daily
  • Target urine pH 6.0-6.5
  • Monitor pH with pH paper
  1. Increased Fluid Intake:
  • Goal urine output >2.5 L/day
  • Low-sugar fluids preferred
  1. Dietary Modifications:
  • Reduce animal protein intake (purine load)
  • Reduce fructose intake
  1. Consider Allopurinol:
  • If hyperuricosuria persists despite alkalinization
  • Also addresses gout history
  1. Metabolic Syndrome Management:
  • Weight loss
  • Glycemic control

Teaching Points

  1. Acidic urine (pH <5.5) is the primary risk factor for uric acid stones
  2. Uric acid stones are radiolucent on X-ray but visible on CT
  3. Metabolic syndrome impairs ammoniagenesis causing persistent low urine pH
  4. Uric acid stones can be dissolved with urinary alkalinization (pH 6.0-6.5)
  5. Target urine output >2.5 L/day for all stone formers

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